Bleaching process



Patented Aug. 23, 1932 UNITED STATES PATENT OFFICE HANS JOHN, OF APPLETON, WISCONSIN, ASSIGNOR TO PAPER PATENTS COMPANY, OF

NEENAH, WISCONSIN, A CORPORATION OF WISCONSm' BLEAGHING PROCESS 1N0 Drawing.

The invention relates to improvements in bleaching and, although certain features of the invention are susceptible of a more extended application, is of particular value in 5 connection with the bleaching of wood pulp used in the manufacture of paper.

In the art of paper making, the bleaching process or bleaching operation is generally understood to be that portion of the entire series of stock preparation steps during which the material is in contact with the socalled bleaching solution, meaning the liquid which is employed to effect certain chemical changes by which the purification and whitening of the pulp is effected, possibly in conjunction with certain subsequent operations such as washing or acidification which follow the actual bleaching operation, but which are not generally understood to be a part of the bleaching operation, nor is the operation of digesting.

The principal objects of the present invention are to provide an improved bleaching process for efi'ecting a high yield of pulp of uniform superior quality; to provide a process which may be readily controlled or manipulated; to provide a process which may be practiced with great economy, speed and precision and, in general, to provide an im- The process will be described as applied to the bleaching of wood pulp made from wood chips which have been previously cooked or digested and the pulp washed and prepared for the bleaching treatment in the customary manner, and in its preferred embodiment comprises the following steps.

. The unbleached pulp is first subjected to the action of a bleach solution containing proved process of the character referred to. I

Application filed November 21, 1827. Serial No. 234,923.

ten times as much available chlorine as is necessary to complete the bleaching operation. In order to prevent loss of the unexhausted excess bleach, the bleach solution at the end of the second or final stage of the process is separated fromthe pul and a portion of it is used for the bleaching of a subsequent batch of pulp in the first bleaching stage, and another, the larger portion of the resulting unexpended bleaching solution, W is first strengthened or fortified and then used in the second stagebleaching of a subsequent batch.

F z'rst stage.The pulp of initially about 25% consistency hasia consistency of about l 10% after the bleach solution has been added and the temperature used inthis stage is about 30 C. The bleach solution" used, is that part of the bleaching solution which remains in the thickened pulp or cake after N draining from the pulp the greater portion of the unexpendedvbleach solution remaining at the end of the secondstage of the treatment. This portion or remainder of the partially exhausted second stage bleach solution will have an original strength of about five grams available chlorine per liter, but in order to extract the same from the cake it will have been necessary to use as much or p0 sibly twice as much water jwhich will corresp ndingly dilute this portion of the unexpendedbleach. In order to maintain v the concentration while at the same time avoiding loss of bleach, a fair compromise will be about one to one dilution which will give to the first stage bleach solution an initial available chlorine strength of about I 2 grams per liter. This figure due to imperfect extraction in practice may be reduced somewhat without serious detriment. Using a solution of the described strength and with the described consistency and temperature the time necessary for the first stage bleaching'would be about ten to thirty minutes, no agitation being necessary except that employed to mix the bleach with the pulp at the commencement of the operation. A

After the first stage of bleaching is completed, water is added to the pulp to bring W it down to about 1% consistency and then tent of the pulp the final mixed solution will have a strength of about 5.6 rams available chlorine per liter which is a out ten times as much bleach strength as is actually needed to complete the bleaching of the ulp during the second stage of the bleac ing process.

During the second stage of the process the pulp is slowly agitated or circulated and the temperature employed is preferably between 20 C. and 35 C. Depending upon the class of pulp treated, about from two to four hours will be required to complete the second bleaching operation.

When the bleaching operation is approaching completion the consumption of chlorine will also practically cease. I Based upon previous experience or the removal of samples for test, the solution will be runoff or filtered with reasonable promptness after the bleaching is complete. This is desirable for the reason that after a certain length of time, if the still potent bleach solution is allowed to remain in contact with the pulp, a degenerative change will occur, resulting in a decomposition of the bleach solution and injury to the fibre. No great exactitude is required in determinin when to separate the pulp, as it is found that this degenerative change does not commence until quite a long time after the end point of the actual bleaching is reached.

At the end of the second bleaching stage the bleach liquid will have a strength of about five grams available chlorine per liter which obviously is too valuable to discard. To recover the same the solution is run off or filtered OE and this part of the unexpended bleach solution is fortified so as to bring it up to the original strength of about six grams per liter so that it can be again used for the second stage bleaching of a subsequent batch of pulp. Such fortification may be effected by returning a portion or all of this liquid to the bleach plant, it being understood that preferably the bleach solution which is employed in the second stage, according to the usual practice, contains calcium hypochlorite and is approximately saturated to calcium hydroxide.

After about 90% of the unexpended bleach solution remaining at the end of the second stage has been filtered off, there will be found remaining in the pulp or"cake about 10% of the same solution which is separated y displacement with water using no more water than is necessary to effect a reasonably complete extraction. As previously stated, a one to one or two to one dilution will not seriously impair the strength or concentration of the bleach solution which, also as previously stated, is used as the bleaching solution in the first stage of the process.

It will be manifest that the process herein described differs from the usual bleaching process in that the bleaching solution or, at least the major portion thereof is used and reused for bleaching successive batches of pulp. This causes the solution to become charged with compounds formed as an incident to the action of the bleaching solution on the pulp and foreign materials carried thereby. Forexample, a substantial amount of calcium chloride is formed during the bleaching operation and this keeps going into solution. However, it is found that because a fraction, in the described example about 10%, of the entire bleaching solution is discarded at the end of the first stage of bleaching, the percentage of soluble reaction compounds in the solution will finally become stabilized and does not reach a value which interferes in anyway with the proper effect of the bleaching solution upon the pulp.

In practicing the process it is obviously not essential to employ great accuracy in adjusting the amount of bleaching solution to the amount of pulp which is being treated, or to the amount of bleaching effect which is required per unit of pulp. Nor is it essential to use great accuracy in fortifying the solution before it is used in the second stage of bleaching, although, as a matter of convenience, it is preferred to standardize the solution to the strength indicated.

Since the only part of the bleaching solution which is discarded is that which has been completely exhausted in partially bleaching the pulp in the first stage of the operation, there is no loss of bleach except the minute percentage left in the final thickened pulp after the bleaching operation is completed. Although an acid condition of the bleaching solution is attained during the progress of the first bleaching step, it will be understood that during this first stage of the bleaching operation the acid condition is not harmful because at this stage the fibres appear to be more or less protected or shielded by the foreign materials which, due to the deficiency in chlorine, have not yet been converted into compounds soluble in an acid solution. As a matter of fact, during the early stage of the bleaching operation an acid reaction is believed to be somewhat desirable in that it causes protective particles of the foreign materials to go into solution, thus allowing the solution to attack other particles directly and hence accelerating the bleaching, in addition to which the dissolved sistency of the plup, obviate the necessity of materials are removed by the washing step at the end of the first stage, and so do not interfere with the bleaching in the second stage.

The use of a large quantity of bleaching solution in the second stage of the process and hence the high dilution or relatively low conviolent agitation so that the fibres are not deprived of theirprotective coating or incrustation of reaction compounds at the time when the bleaching process is being completed and when the fibres are quite susceptible to deleterious influences. tinued alkalinity of the solution during the end stage of the process and because of the substantial absence of loss or damage due to interfriction of the fibres, the yield of cellulose of the highest quality will be very high. Such pulp contains a very large percentage of alpha cellulose having a low KOH solubility, a low copper number and shows little if any strength loss by bleaching as compared with the unbleached pulp. In addition to the ease of control above referred to, the time consumption of the process herein described compares favorably with the ordinary bleaching process. Also, there is a substan-- tial economy in power and steam, and the product is very uniform.

After the actual bleaching operation has .been completed, and after the pulp has been separated from the bleaching solution, the residual compounds are removed by first treating the pulp with SO water which contains .05% of SO gas dissolved therein. This treatment converts into soluble form those residual compounds which did not dissolve in the alkaline bleach solution. The pulp is then given a further washing treatment with water and is concentrated in the usual manner ready for use. Ordinarily, it will be found that the acid wash or SO treatmentcan be completed in about one hour with a moderate circulation when the consistency of the stock is about 2%. It is not necessary to employ artificial heat during the acid or washing treatment.

For convenience, th process has been described as applied to' a true batch method. However, it will be readily understood by those skilled in the art that the process may be susceptible of use in connection with continuous manipulation. Hence the term batch as used herein is intended to include that portion of the entire supply of pulp which is being at that time subjected to the specified treatment step whether in a' continuous or atrue batch svstem. The same remarks obviously apply in respect of the words stepl, or stage as used herein.

Therefore, it will be understood that the described details of the process are merely descriptive of a single specific application of the invention, the scope of which should be de- Thus because of the con-' termined by reference to the appended claims, said claims being construed as broadly as possible, consistent with the state of the art.

I claim as my invention:

1. The improvement in the art of bleach ing paper pulp which consists in partially bleaching the stock, completing the bleaching operation by subjecting a batch of the partially bleached pulp to the action of a solution containing (a) a. plurality of times as much bleaching agent as is necessary to complete the bleaching operation, and (b) a sufficient amount of alkaline material to prevent the development of an acid reaction at the completion of the bleaching operation.

2. The improvement in the art of bleaching paper pulp which consists in partially.

bleaching the stock. completing the bleaching operation bv subjecting a batch of the partially bleached pulp to the action of a solution containing (a) a plurality of times as much bleaching agent as is necessary to complete the bleaching operation. and (b) a sufiicient amount of alkaline material to prevent the development of an acid reaction at the completion of the bleaching operation and, after the bleaching operation has been completed. recovering a substantial proportion of the partiallv exhausted bleaching solution for use in bleach ng a subsequent batch.

3. The improvement in the art of bleaching paper pulp which consists in partially bleaching the stock. completing the bleaching operation by subjecting a batch of the partially bleached pulp to the action of a solution containing (a) a plurality of times as much bleachins. agent as is necessarv to complete the bleaching operation. and (b) asuflicient amount of alkaline material to prevent the development of an acid reaction at the completion of the bleaching operation and. after the bleaching operation has been completed, recovering a substantial proport on of the part allv exhausted bleaching solution for use in bleaching a subsequent batch and then restorin the recovered bleaching solution to substantially its original strength.

4. The improvement in the art of bleaching paper pulp which consists in partially bleaching the stock completing the bleaching operation by subjecting the partially bleached stock to the action of a bleaching solution in which the pulp represents not more than 3 percent by weight. such solution containing a plurality of times as much bleaching agent as is necessary to complete the bleaching operation.

5. The improvement in the art of bleach ng paper pulp which consists in partiallv bleaching the stock. completing the bleaching operation by subiecting the partially bleached stock to the action of a bleaching solution in which the pulp represents not more than three percent by weight, su'ch solution containing a plurality of times as much bleaching agent as is necessary to complete the bleaching operation, and also containing a sufiicient amount of alkaline material to prevent development of an acid reaction at the end of said bleaching operation.

6. The improvement in the art of bleaching paper pulp which consists in partially bleaching the stock, completing the bleaching operation by subjecting the partially bleached stock to the action of a bleaching solution in which the pulp represents not more than three percent by weight, such solution containing a plurality of times as much bleaching agent as is necessary to complete the bleaching operation, and also containing a suflicient amount of alkaline material to prevent development of an acid reaction at the end of said bleaching operation, and, after the bleaching operationv has been completed, separating a substantial proportion of the said solution from the pulp and restoring it to substantially its original strength for use in bleaching a subsequent batch.

7. The improvement in the art of bleaching paper pulp which consists in partially bleaching the stock, completing the bleaching operation by subj ecting the pulp to the action of a solution containing a plurality of times as much bleaching agent as is necessary to complete the bleaching operation and including a sufiicient amount of alkaline material to prevent the development of an acid reaction at the completion of the bleaching operation and, after the bleaching is complete but be-, fore degeneration occurs, separating the excess bleach from the pulp.

8. The improvement in the art of bleaching paper pulp which consists in subjecting the pulp to the action of a solution contain ing a plurality oftimes as much bleaching agent as is necessary to complete the bleaching operation and also a suflicient amount of alkaline material to prevent the development of an acid reaction at the completion. of the bleaching operation and, after the consumption of bleach is at a standstill and before there is any material degeneration of the valuable materials present, separating the pulp from the solution and utilizing a substantial proportion of the latter in a subsequent bleaching operation.

9. The improvement in the art of bleach-, ing paper pulp which consists in first treating the batch with a solution containing only a suflicient quantity of bleaching agent to partially bleach the pulp by the time that the bleaching value of the solutibn is exhausted, then washing the pulp and then completing the bleaching operation by' subjecting the partially bleached pulp to the action of a solution containing bleaching agent in amount greatly in excess of that necessary to complete the bleaching operation.

10. The improvement in the art of bleaching paper pulp which consists in first treat- I ing the batch with a solution containing only suflicient bleach to partially bleach the pulp by the time that the bleaching value of the solution is exhausted, then washing the pulp, then completing the bleaching operation by subjecting the partially bleached pulp to the action of a solution containing bleaching agent in amount greatly in excess of that necessary to complete the bleaching operation, and then, after the completion of the bleaching operation, recovering a substantial proportion of the said excess bleach for use in bleaching a subsequent batch.

11. The improvement in the art of bleaching paper pulp which consists in first treating the batch with a solution containing only suflicient bleach to partially bleach the pulp by the time that the bleaching value of the solution is exhausted, then washing the pulp, then completing the bleachin operation by subjecting the partially bleached pulp to the action of a solution containing bleaching agent in amount greatly in excess of that necessary to complete the bleaching operation. and then, after the completion of the bleaching operation, recovering a substantial proportion of the said excess bleach for use in completing the bleaching of a subsequent batch.

12. The improvement in the art of bleaching paper pulp which consists in first treating the batch with a solution containing only sufiicient bleach to partially bleach the pulp by the time that the bleaching value of the solution is exhausted, then washing the pulp, then completing the bleaching operation by subjecting the partially bleached pulp to the action of a solution containing bleaching agent in amount greatly in excess of that necessary to complete the bleaching operation, then, after the completion of the bleaching operation, recovering a substantial proportion of the said excess bleach for use in completing the bleaching of a subsequent batch, and in using another portion of the said excess in the first stage bleaching of a subsequent batch.

13. 'The improvement in the art of bleaching paper pulp' which consists in first subjecting a batch of pulp to the action of a bleaching solution containing insuflicient bleach to complete the bleaching operation, then washing the pulp by dilution of water and subsequent thickening said pulp and then completing the bleaching operation by subjecting the partially bleached pulp to the action of a solution containing the pulp in a concentration of not more than about three percent by weight and bleaching agent in amount a plurality of times that which is necessary to complete the bleaching operation and then, after the bleaching operation has been completed, separating the partially exhausted bleaching solution from the pulp ISO and utilizing a-portion' thereof in the preliminary bleaching of a subsequent batch and a portion thereof for completing the bleaching of a subsequent batch.

14. The improvement in the art of bleaching paper pulp which consists in first subjecting a batch of pulp to the action of a bleaching solution containing insutlficient bleach to complete the bleaching operation, then washing the pulp by dilution of water and subsequent thickening said pulp and then completing the bleaching operation by subjecting the partially bleached pulp to the action of a solution containing te pul in .a concentration of not more than about t ree percent by weight aud'bleaching agent in amount a plurality of times that which is necessary to complete the bleaching operation and then, after the bleaching operation has been completed, separating the partially exhausted bleachihg solution from the pulp and utilizing aportion thereof in the preliminary bleaching of a subsequent batch and a portion thereof for completing the bleaching of a subsequent batch, and restoring said last named portion to substantially its original strength.

15. A process according to the preceding claim, in which the bleaching solution used in the second stage contains suflicient alkaline material to prevent the development of an acid reaction at the end of said second stage.

16. The improvement in the art of bleaching which consists in subjecting the material to the action of a bleaching agent until the bleaching is only partially complete, while ermitting an acid reaction during said rial, and then subjecting the material to the action of a bleaching agent in a bath which is maintained alkaline throughout said bleaching stage.

17. The improvementv in the art of bleaching which comprises partially bleaching the stock, rinsing the same, and then completing the bleaching thereof in a bath containing a large excew of the same bleaching agent as that used in the first stage of, the bleaching operation, and subsequently recovering the excess of bleaching agent after the second stage and using a portion of it in the first stage and another portion in the second stage of a subsequent bleaching operation.

18. The improvement in the art of bleach ing which comprises partially bleaching the stock,,rinsing the same, and then completing the bleaching thereof in a bath containing-a large excess of the same bleaching agent as that used in the first stage of the bleaching operation, and subsequently recovering the excess of bleaching agent after the second stage and using a portion of it in the first stage and another portion, fortified with additional bleaching agent, in the second stage of a subsequent bleaching operation.

leaching operation, then washing the mate- 19. The improvement in the art of bleaching which comprises partially bleaching the stock, rinsing the same, and then completing the bleaching thereof in a bath containing a large excess of the bleaching agent, and subsequently recovering the excess of bleaching agent after the second stage and using a portion of it in the first stage and another portion in the second stage of a subsequent bleaching operation, said second stage bath containing sufficie-nt alkaline material to prevent the dev velopment of an acid reaction at the comple-" of asubsequent bleaching operation, the bath' in the second stage containing sufficient alka-' line material to prevent the development of an acid reaction at the completion of the bleaching operation.

21. A process as defined in claim 14 wherein the bleaching solution used in the second stage contains suflicient alkaline material to prevent the development of an acid reaction at the end of said second stage, said bleaching solutlon comprising a compound of calcium and chlorine and said alkaline material comthe bleaching, and wherein the bleaching agent in both stages contains active chlorine. u

24. A process as defined in claim 16 wherein calcium hydroxide is used to maintain the second bleaching bath alkaline.

' 25. A process as defined in claim 16 wherein the second bleaching bath initially contains a plurality of times the quantity of bleaching agent that will be consumed in said second stage. v

26. A process as defined in claim 16 wherein the second bleaching bath initially contains a plurality of times the quantity of bleaching agent that will be consumed in said second stage, wherein the bleaching agent in both stages contaiils active chlorine, and whereincalcium hydroxide is used to maintain the second bleaching bath alkaline.

DR. HANS JOHN. ,m

CERTIFICATE OF CORRECTION.

rmm- No. 1,872, 743. August 23, 1932.

HANS JOHN.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, line 42, claim 16, before "bleaching" insert the word second; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office;

Signed and sealed this 11th day of October, A. D, 1932.

M. J. Moore, (Seal) Acting Commissioner of Patents. 

